Interestingly not only did Darwin make the above observations leading up to his publication of "Origin of the Species" but so did another naturalist (a creationist and missionary) half a world away during the same period!
People often talk about "Darwin's idea" but forget that Alfred Russell Wallace also observed the same things and came to the same conclusions.
In fact when he wrote to Darwin in order to discuss his findings, it so shocked Darwin that it nearly discouraged him from ever publishing his own observations.
Fortunately Darwin's friends encouraged him to publish His own & Wallace's findings in a joint paper presented at the Royal Society.
The fact that two men observed and collected the same facts in different parts of the world and came to the same conclusions highlights the inherent logic, parsimony (and inevitability) of the theory of common descent.